In many applications of electro-technology, materials of different electrical properties are combined. The distribution of electric field in and around an electric device depends on the electric properties of the materials used in the device, as well as on device geometry. In AC applications, the field distribution depends to a large extent on the permittivities of the materials of the device, while in DC applications, the field distribution is to a large extent dependent on the conductivity of the device material.
In many devices, different materials showing very distinct electrical/dielectric properties are in contact. In such devices, the equipotential lines of the electric field tend to concentrate at the interfaces towards the low permittivity or low conductivity regions. The larger the difference between permittivities/conductivities of different materials which are in contact, the more uneven the field distribution becomes. Such concentration of the electric field increases the risk of electrical breakdown, and the insulating properties of the material is thus impaired.